1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
An image forming apparatus using an electrophotographic method forms an image by developing a latent image formed on a photoreceptor to form a toner image and then, transferring and fixing the toner image onto a recording medium such as paper. As a fixing method, a heat-pressure fixing method is generally used.
The heat-pressure fixing method is a method of pressing and heating the recording medium onto which a toner image has been transferred and not yet fixed, through a pressure-contact area between a pair of rollers which rotate in pressure-contact with each other and at least one of which is heated. Upon making the pressing and heating, a main component of toner composition, i.e., thermoplastic resin is fused and softened so as to be adhered to the recording medium, thus being fixed as a toner image onto the recording medium.
Properties desired for an image forming apparatus include higher speed and availability for various kinds of recording mediums. In the heat-pressure fixing method, however, fixing speed is limited based on heat quantity available for a recording medium and therefore, the number of sheets having images formed thereon is also limited per unit of time during a continuous image forming operation. And what is worse, in a fixing process for recording medium such as heavy paper, the heat quantity to be supplied to the recording medium must be set to be larger, or alternatively, fixing speed must be set to be lower, compared to those in a fixing process for a recording medium such as plain paper.
In view of the above matters, suggestions have been made to preheat recording mediums having toner images transferred thereto, up to a predetermined temperature (in an approximate range of 100° C. to a toner softening temperature) before the recording mediums are subjected to a fixing process so that differences among fixing conditions depending on recording medium-related information including a type, size, or the like element of the recording mediums are canceled out each other, thereby preventing image quality from being degraded.
In the case just stated, another problem arose such that when the toner image is heated on the recording medium to be fixed thereto by pressure, a larger amount of toner attached onto the recording medium causes the toner to scatter even to edges and then be fixed thereto, resulting in rough edges and rough image boundaries, which result represents degraded image quality.
The problem as above became more prominent as a toner had smaller particles and was more spherical along with an increasing demand for higher-quality images and higher-resolution images.
In a technique disclosed by Japanese Unexamined Patent Publication JP-A 9-160408 (1997), a temperature of a thermal heater in a preheating section is switched to a predetermined temperature depending on a material of recording medium inputted to a control unit, and information of detected temperature of a belt member is allowed to be accurately fed back to the control unit owing to a temperature sensor installed in the thermal heater, thus achieving such a configuration that toner images transferred onto recording mediums different in material and thickness are all heated to a certain preliminary temperature, with the result that high-quality images can be recorded on various transfer materials.
In a technique disclosed by Japanese Unexamined Patent Publication JP-A 2003-271007, a fixing device of heat-pressure fixing method is provided with a preheating section for preheating a recording medium having a toner image not yet fixed thereto so that a temperature of the recording medium entering a pressure-contact area between rollers falls in a predetermined temperature range, whereby a proper amount of heat is applied to the recording medium in the fixing process, not depending on a type, size, or the like element of the recording medium, thus being capable of forming a fixed image of high quality.
In a technique disclosed by Japanese Unexamined Patent Publication JP-A 2006-154244, a fixing device having a preheating section for preheating a recording medium before a fixing process, uses a polymerized toner having a two-layered core-shell structure of which shell part is made of resin that is softened when preheated, thereby causing a toner-to-toner attachment which eliminates scattering and hovering of the toner whereby no such phenomena occur as toner scattering and toner tailing in the fixing process, with the result that a high-quality and high-resolution image can be formed.
In the technique disclosed by JP-A 9-160408, the preliminary temperature is maintained in the range of 100° C. or more and a toner softening temperature or less, and the toner on the recording medium is preheated to a temperature around its softening temperature. It is therefore inferred that toner-to-toner fusion may partly occur, but in the case where the amount of toner attached to the recording medium is large, the toner cannot be completely prevented from scattering.
In the technique disclosed by JP-A 2003-271007, the preliminary temperature is low, about 100° C. or less, so that the preheating process does not cause toner fusion. The toner therefore scatters, thus degrading image quality, especially in the case where the amount of toner attached to the recording medium is large.
In the technique disclosed by JP-A 2006-154244, an amount of toner attached to the recording medium is not considered and therefore, in the case where the amount of toner attached to the recording medium is large, heat is transferred to not an entire toner layer during the preheating process, resulting in insufficient toner-to-toner fusion which deteriorates image quality when the toner is prevented from scattering. In addition, since the technique disclosed by JP-A 2006-154244 relates to the polymerized toner which is substantially spherical, such a shape causes the toner particles to easily come off from a toner layer before the preheating step, thus causing the toner to scatter.